The present invention relates to a method for making a electroconductive pattern in an organic polymer layer which is suitable as an electronic circuitry in an electric or semiconductor device.
Electric or semiconductor devices such as flat panel displays, photovoltaic cells or electrochromic windows typically contain a substrate provided with an indium tin oxide (ITO) layer as a transparent electrode. The coating of ITO is carried out by vacuum sputtering methods which involve high temperature conditions up to 250xc2x0 C., and therefore, glass substrates are generally used. The high cost of the fabrication methods and the low flexibility (pliability) and stretchability of such electrodes, due to the brittleness of the inorganic ITO layer as well as the glass substrate, limit the range of potential applications. As a result, there is a growing interest in making all-organic devices, comprising plastic resins as a substrate and organic electroconductive polymer layers as an electrode. Such plastic electronics allow to obtain low cost devices with new properties (Physics World, March 1999, p.25-39). Flexible plastic substrates can be provided with an electroconductive polymer layer by continuous roller coating methods (compared to batch process such as sputtering) and the resulting organic electrodes enable the fabrication of electronic devices characterised by a higher flexibility and a lower weight.
The production and the use of electroconductive polymers such as polypyrrole, polyaniline, polyacetylene, polyparaphenylene, polythiophene, polyphenylenevinylene, polythienylenevinylene and polyphenylenesulfide are well known in the art. EP-A-440 957 describes a method for preparing polythiophene in an aqueous mixture by oxidative polymerisation in the presence of a polyanion as a doping agent. In EP-A-686 662 it has been disclosed that highly conductive layers of polythiophene, coated from an aqueous coating solution, could be made by the addition of a di- or polyhydroxy and/or a carbonic acid, amide or lactam group containing compound in the coating solution of the polythiophene layer and by keeping the coated layer at elevated temperature, preferably between 100 and 250xc2x0 C., during preferably 1 to 90 seconds.
Coated layers of organic electroconductive polymers can be structured into patterns using the known wet-etching microlithography techniques. WO97/18944 describes a process wherein a positive or negative photoresist is applied on top of a coated layer of an organic electroconductive polymer, and after the steps of selectively exposing the photoresist to UV light, developing the photoresist, etching the electroconductive polymer layer and finally stripping the non-developed photoresist, a patterned layer is obtained. A similar technique has been described in Synthetic Metals, 22 (1988), p. 265-271 for the design of an all-organic thin-film transistor. Such methods are cumbersome as they involve many steps and require the use of hazardous chemicals. Research Disclosure No. 1473 (1998) describes photoablation as a method suitable for patterning organic electroconductive polymer layers, wherein the selected areas are removed from the substrate by laser irradiation. Such photoablation processes are convenient, dry, one-step methods but the generation of debris still requires a wet cleaning step and may contaminate the optics and mechanics of the laser structuring device. Some prior art methods also induce a difference of the optical density between electroconductive and non-conductive areas of the patterned surface, which should be avoided.
It is an object of the present invention to provide a convenient, dry, one-step method of patterning an electroconductive organic polymer layer which does not require a wet cleaning step and which does not influence the optical density of the polymer layer. This object is realized by the method of claim 1. Specific features for preferred embodiments of the invention are disclosed in the dependent claims.
Further advantages and embodiments of the present invention will become apparent from the following description.
All values of surface resistivity (SR) presented in this document are measured according to the following method. The sample is cut to obtain a strip having a length of 27.5 cm and a width of 35 mm. Electrodes are applied over the width of the strip at a distance of 10 cm from each other. The electrodes are made of a conductive polymer, ECCOCOAT CC-2 available from Emerson and Cumming Speciality polymers. A constant potential is then applied over said electrodes and the current flowing through the circuit is measured on a Pico-amperemeter KEITHLEY 485. SR in xcexa9/square (xcexa9/xe2x96xa1) is calculated from the measured values of potential and current, taking into account the geometry of the measurement.
The polythiophene layer used in the method of the present invention is similar to those described in EP-A-686 662. However, contrary to the method of EP-A-686 662, the coated layer is not dried or treated at elevated temperature, so that the initial surface resistivity (SRi) is kept at a high value of at least 104 xcexa9/xe2x96xa1, or more preferably at least 106 xcexa9/xe2x96xa1, i.e. the conductivity of the layer is insufficient to be suitable as an electrode. By heating selected areas (paths), hereinafter referred to as xe2x80x98image-wise heatingxe2x80x99, the SR of said areas is decreased to SRi/xcex94, wherein the factor xcex94 is at least 10, preferably at least 103 or even more preferably at least 105 so as to obtain a pattern of conductive and non-conductive areas which can be used as an electronic circuitry. Said electronic circuitry can be a relatively low-resolution circuitry as typically used in printed circuit boards or a high-resolution microcircuitry as used in integrated semiconductor devices. More applications of the present invention are indicated below.
Simulation experiments, wherein the above mentioned polymer layer is overall heated in an oven, indicate that a treatment at 200xc2x0 C. during 2 minutes suffices to reduce SRi by a factor xcex94 between 104 and 105. The thickness of the layer is not significantly influenced by such treatment which illustrates that the organic polymer layer can be rendered electroconductive by heat treatment without invoking ablation. So the method of the present invention is particularly distinguished from prior art methods in that no material is removed from the layer, i.e. the conducting and non-conducting areas are all located in the same plane which may be beneficial when used in devices wherein very thin layers are applied on the electrodes, e.g. organic light emitting polymer displays. Moreover, the method of the present invention allows to obtain an electroconductive pattern showing no substantial difference of optical density between conducting and non-conducting areas. As an alternative method, contrary to the method of the present invention, one may also obtain an electroconductive pattern by ablating selected areas of the polymer layer and then overall heating the remaining areas, thereby decreasing the SR value so as to render said remaining areas electroconductive. However, said alternative method also involves ablation and thus is characterized by the same problems as the prior art methods described above.
The polythiophene used in the method of this invention has preferably the following formula: 
in which n is larger than 1 and each of R1 and R2 independently represents hydrogen or an optionally substituted C1-4 alkyl group or together represent an optionally substituted C1-4 alkylene group or an optionally substituted cycloalkylene group, preferably an ethylene group, an optionally alkyl-substituted methylene group, an optionally C1-12 alkyl- or phenyl-substituted ethylene group, a 1,3-propylene group or a 1,2-cyclohexylene group.
The preparation of such a polythiophene and of aqueous dispersions containing such a polythiophene and a polyanion is described in EP-A-440 957 and corresponding U.S. Pat. No. 5,300,575. Basically the preparation of polythiophene proceeds in the presence of polymeric polyanion compounds by oxidative polymerisation of 3,4-dialkoxythiophenes or 3,4-alkylenedioxythiophenes according to the following formula: 
wherein R1 and R2 are as defined above.
Stable aqueous polythiophene dispersions having a solids content of 0.05 to 55% by weight and preferably of 0.1 to 10% by weight can be obtained by dissolving thiophenes corresponding to the formula above, a polyacid and an oxidizing agent in an organic solvent or preferably in water, optionally containing a certain amount of organic solvent, and then stirring the resulting solution or emulsion at 0 to 100xc2x0 C. until the polymerization reaction is completed. The polythiophenes formed by the oxidative polymerization are positively charged, the location and number of such positive charges being not determinable with certainty and therefore not mentioned in the general formula of the repeating units of the polythiophene polymer.
The oxidizing agents are those which are typically used for the oxidative polymerization of pyrrole as described in for example J. Am. Soc. 85, 454 (1963). Preferred inexpensive and easy-to-handle oxidizing agents are iron(III) salts, e.g. FeCl3, Fe(ClO4)3 and the iron(III) salts of organic acids and inorganic acids containing organic residues. Other suitable oxidizing agents are H2O2, K2Cr2O7, alkali or ammonium persulfates, alkali perborates, potassium permanganate and copper salts such as copper tetrafluoroborate. Air or oxygen can also be used as oxiding agents. Theoretically, 2.25 equivalents of oxidising agent per mol of thiophene are required for the oxidative polymerization thereof (J. Polym. Sci. Part A, Polymer Chemistry, Vol. 26, p.1287, 1988). In practice, however, the oxidizing agent is used in excess, for example, in excess of 0.1 to 2 equivalents per mol of thiophene.
The polyacid forms a polyanion or, alternatively, the polyanion can be added as a salt of the corresponding polyacids, e.g. an alkali salt. Preferred polyacids or salts thereof are polymeric carbonic acids such as poly(acrylic acid), poly((meth)acrylic acid) and poly(maleic acid) or polymeric sulphonic acids such as poly(styrene sulphonic acid) or poly(vinyl sulphonic acid). Alternatively, copolymers of such carbonic and/or sulphonic acids and of other polymerizable monomers such as styrene or acrylates can be used. Poly(styrene sulphonic acid) is especially preferred. The molecular weight of these polyanion forming polyacids is preferably between 1000 and 2xc3x97106, more preferably between 2000 and 5xc3x97105. These polyacids or their alkali salts are commercially available and can be prepared according to the known methods, e.g. as described in Houben-Weyl, Methoden der Organische Chemie, Bd. E20 Makromolekulare Stoffe, Teil 2, (1987), pp. 1141.
The polythiophene dispersions obtained according to the above method can then be used as basic ingredient of a coating solution. The coating solution can also comprise additional ingredients, such as one or more binders, one or more surfactants, spacing particles, UV-filters or IR-absorbers. Suitable polymer binders are described in EP-A 564 911. Such binders may be treated with a hardening agent, e.g. an epoxysilane as described in EP-A 564 911, which is especially suitable when coating on a glass substrate.
The coating solution can be applied to the substrate by any means known in the art: it can be spin-coated, sprayed or coated by any of the continuous coating techniques that are used to coat solutions on running webs, e.g. dip coating, rod coating, blade coating, air knife coating, gravure coating, reverse roll coating, extrusion coating, slide coating and curtain coating. An overview of these coating techniques can be found in the book xe2x80x9cModern Coating and Drying Technologyxe2x80x9d, Edward Cohen and Edgar B. Gutoff Editors, VCH publishers, Inc, New York, N.Y., 1992. It is also possible to coat simultaneously multiple layers by coatings technique such as slide coating and curtain coating. It is also possible to apply the coating solution to the substrate by printing techniques, e.g., ink-jet printing, gravure printing, flexo printing, or offset printing.
The coating solution is preferably applied to the substrate in such an amount that the coated polymer layer contains between 10 and 5000 mg of polythiophene per m2, more preferably between 100 and 500 mg of polythiophene per m2. The coated polymer layer is preferably dried at a temperature below 100xc2x0 C., more preferably below 70xc2x0 C. and even more preferably between 40xc2x0 C. so that the SR of the polymer layer is at least 104 xcexa9/xe2x96xa1, or more preferably at least 106 xcexa9/xe2x96xa1.
The coated polymer layer further comprises a di- or polyhydroxy organic compound, which can be added to the coating solution. Preferred examples of such di- or polyhydroxy organic compounds are sugar or sugar derivatives such as arabinose, saccharose, glucose, fructose and lactose, or di- or polyalcohols such as sorbitol, xylitol, mannitol, mannose, galactose, sorbose, gluconic acid, ethylene glycol, di- or tri(ethylene glycol), 1,1,1-trimethylol-propane, 1,3-propanediol, 1,5-pentanediol, 1,2,3-propanetriol, 1,2,4-butanetriol, 1,2,6-hexanetriol, or aromatic di- or polyalcohols such as resorcinol. The amount of these compounds in the coated layer may be between 10 and 5000 mg/m2, preferably between 50 and 1000 mg/m2.
The substrate used in the method of the present invention can be inorganic or organic. Suitable polymeric films are made of e.g. a polyester such as poly(ethylene terephthalate), poly(ethylene naphthalate), polystyrene, polyethersulphone, polycarbonate, polyacrylate, polyamide, polyimides, cellulosetriacetate, polyolefines, polyvinylchloride, etc. As inorganic substrates can be used silicon, ceramics, oxides, glass, polymeric film reinforced glass, glass/plastic laminates, e.g. as described in WO 99/21707 and WO 99/21708. The latter is a laminate of a flexible, thin glass and a plastic support, which is a highly suitable substrate for use in the present invention since such a material provides both a good dimensional stability as well as some flexibility. The substrate is preferably provided with an adhesion improving layer, whereon the above described coating solution can be applied.
The step of image-wise heating the polymer layer is preferably carried out by means of a scanning device such as a laser or a line-wise heating device such as a thermal head. Also other devices can be used, e.g. a heated stamp. The energy applied to the polymer layer during image-wise heating should be sufficiently high to decrease the SR value of the polymer layer so as to obtain electroconductive areas, but should not exceed a treshold value above which ablation or chemical processes are capable of substantially destroying the polymer layer.
Typical lasers which can be used in the method of the present invention are e.g. a He/Ne or Ar laser. Preferably, a laser emitting near infrared light having a wavelength in the range from about 700 to about 1500 nm is used, e.g. a semiconductor laser diode, a Nd:YAG or a Nd:YLF laser. The required laser power depends on the pixel dwell time of the laser beam, which is dependent on the scan speed (e.g. between 0.1 and 20 m/s, preferably between 0.5 and 5 m/s) and the spot diameter of the laser beam (defined at 1/e2 of maximum intensity e.g. between 1 and 100 xcexcm, preferably between 10 and 25 xcexcm). More details of the specific exposure parameters are given while discussing the examples below.
In order to improve the light absorption, a compound which is capable of converting light into heat may be added to the polymer layer. Useful compounds are for example organic dyes, carbon black, graphite, metal carbides, borides, nitrides, carbonitrides, or oxides. The polythiophene itself absorbs infrared light, so it may be unnecessary to add dyes or pigments when an infrared light source is used. The addition of a dye or pigment capable of absorbing visible light should even be avoided when the method of the present invention is to be used for making a transparent electrode. Alternatively, the support may absorb the incident light, e.g. when exposing a plastic support such as poly(ethylene terephthalate), to an excimer laser.
A typical thermal head for use in the method of the present invention contains a plurality of adjacent, microscopic heat-resistor elements, which convert electrical energy via the Joule effect into heat. Such thermal printing heads may be used in contact or, more preferred, close proximity with the polymer layer so as to transfer efficiently the heat to the polymer layer. The operating temperature of common thermal printheads is in the range of 300 to 400xc2x0 C. and the heating time per element may be less than 20 ms or even less than 1.0 ms, the pressure contact of the thermal printhead with the material being e.g. 200 to 500 g/cm2 to ensure a good transfer of heat.
The electroconductive pattern obtained by the method of the present invention can be used as an electronic circuit for making an electric or semiconductor device such as a printed circuit board, an integrated circuit, a display, an electroluminescent device or a photovoltaic cell. Preferred examples of a display are passive-matrix liquid crystal displays (LCDs) as well as active-matrix LCDs such as thin-film-transistor (TFT) displays. Particular examples are twisted nematic (TN), supertwisted nematic (STN), double supertwisted nematic (DSTN), retardation film supertwisted nematic (RFSTN), ferroelectric (FLC), guest-host (GH), polymer-dispersed (PF), polymer network (PN) liquid crystal displays. Emissive flat panel display types which may benefit from the present invention are e.g. plasma displays (PDs), field emission displays (FEDs) and so-called organic light-emitting polymer displays (OLEDs).
In OLEDs electrons and holes are injected from a cathode and anode respectively into an electroluminescent material, e.g. an electroluminescent polymer such as poly(p-phenylenevinylene) (PPV) and its derivatives, and then recombine to an exciton which relaxes to the ground state by radiative decay. Such an OLED typically comprises the following layers:
a reflecting cathode, e.g. a low-work function metal layer such as evaporated Ca.
an electroluminescent layer, e.g. PPV; other suitable electroluminescent compounds are described in e.g. xe2x80x9cOrganische Leuchtdiodenxe2x80x9d, Chemie in unserer Zeit, 31. Jahrg. 1997, No.2, p.76-86.
an hole-injection layer
an transparent anode
a transparent substrate
The hole-injection layer as well as the anode layer can be patterned layers obtained according to the method of the present invention.
The patterned electrode obtained by the method of the present invention can also be used for screening electromagnetic radiation or earthing electric charges, for making touch screens, radio frequency identification tags, electrochromic windows and in imaging systems, e.g. silver halide photography or electrophotography. Also a device such as the electronic book described in WO 97/04398 may particularly benefit from the flexible electrode of the present invention. Even more applications are described in WO 97/18944.